The invention is concerned with catalysts for the isomerisation of straight hydrocarbons into branched hydrocarbons which therefore have a better octane rating.
These isomerisation reactions are of considerable importance industrially. In fact, internal combustion engines require fuels with a high octane rating. This high rating used to be obtained by adding tetraethyl lead to the petroleum. To reduce lead pollution, the tetraethyl lead is replaced by other products with a high octane rating, either by aromatics which are also harmful to the environment or by tertiary ethers. These products make up the composition of unleaded petrol.
By way of illustration, the octane rating of isooctane (2-dimethyl 3-methylpentane) is, by definition, 100, and that of n-heptane is 0. A naphtha C.sub.5 -C.sub.9 petroleum cut has an octane rating of 70, which, after gas reforming, is about 91, and, after tetraethyl lead has been added, is 94. Pure toluene has a rating of 97.
Avoiding tetraethyl lead and aromatics in fuels has meant that a great interest has been taken in finding isomerisation processes which enable branched isomers to be obtained from straight aliphatic hydrocarbons, the octane rating of which branched isomers is higher than that of their straight counterparts.
An isomerisation reaction of this kind is well-known to the skilled person: it is carried out by passing the hydrocarbon to be isomerised and which is diluted in hydrogen over a suitable catalyst at a temperature close to 350.degree. C. At the outlet from the reactor a mixture is collected of different isomers which are branched to varying degrees, of cracked products which therefore contain fewer carbon atoms than tile starting hydrocarbon, and of cyclic products. The efficiency of the reaction can be assessed by the percentage of molecules of the starting hydrocarbon which are transformed into branched hydrocarbons, cracked products of cyclic products being regarded as undesirable.
The catalysts usually used are of the bifunctional kind: a dehydrogenating-hydrogenating function provided by precious metals, platinum-rhenium or platinum-irridium, and an isomerising acid function provided by the support, usually gamma aluminium doped with chlorine. Other catalysts then appeared which were composed of platinum with a zeolite support. Finally, more recently, a new class of catalysts has been found which was based on totally different chemical types since they were catalysts with a base of heavy metal carbides. These catalysts and the processes for their preparation are described in the following patent applications:
EP-A- 0 396 475 (PECHINEY ELECTROMETALLURGIE): Obtaining heavy metal carbides with a high specific surface area.
This application describes a process for obtaining heavy metal carbides with a high specific surface area, characterised in that a gaseous compound of said metal is reacted with a reactive carbon with a specific surface area of at least 200 m.sup.2 /g at a temperature of between 900.degree. C. and 1400.degree. C., and the carbides thus obtained.
The examples in this application compare the activity of this new type of catalyst, particularly the catalyst with a base of tungsten and molybdenum carbides, with a conventional catalyst, Al.sub.2 O.sub.3 +0.25% platinum, for the isomerisation of methylcyclopentane and of n-hexane.
EP-A- 0 440 569 (PECHINEY ELECTROMETALLURGIE): Process for obtaining porous solid bodies with a base of refractory carbide by means of organic compounds and metal or metalloid compounds.
This application describes a process for obtaining porous solid bodies of a shape and porosity to suit requirements, of a carbide with a high specific surface area, characterised in that a carbonisable compound which is exclusively organic, polymeric and/or copolymerisable and capable of giving a carbonaceous solid skeleton is mixed with a metal or metalloid powder or one of its compounds which is reducible by carbon, the mixture is shaped, the organic compound is cross-liked or hardened, and is subjected to a heat treatment for the organic compound to be carbonised at between 500.degree. and 1000.degree. C., and then for carburation to be carried out.
EP-A- 0 474 570 (GIE PECHINEY RECHERCHE): A process for activation from the surface of carbides of heavy metals with a high specific surface area for the purpose of catalytic reactions.
This application describes a process for activation of carbides of heavy metals with a high specific surface area in order for them to be used as catalysts in chemical or petrochemical reactions, consisting in subjecting the carbides to an oxidation treatment which is carried out in a current of oxidising gas at a temperature of between 250.degree. and 450.degree. C. by keeping a temperature stage of at least 3 hours, then by cooling to ambient temperature, still in the presence of an oxidising current. Example 1 shows the comparison between a catalyst with a base of molybdenum carbide which is activated in accordance with the process described and a conventional catalyst with a Pt base for isomerisation of the n-hexane.
EP-A- 0 511 919 (coating) PECHINEY ELECTROMETALLURGIE): Catalytic system, in particular for post-combustion of the exhaust gases and the process for their production.
This application describes a catalytic system which is made up of a support on which the catalytically active product is deposited. The support has worthwhile mechanical or physical properties for the required operating conditions, but a mediocre specific surface area. The catalytically active product, a metallic carbide, is obtained by coating the support in a suspension of a reducible compound of the metal in a solution of an organic compound, carbonisation of that compound, reduction of the metallic compound, and carburation of the metal. The carbide thus obtained has a high specific surface area.
Preferably, the support is constituted by silicon carbide prepared by carbonisation of a paste containing silicon, carbon and an organic resin. In the examples, the catalytically active carbide is a molybdenum, iron, tungsten, or vanadium carbide.
EP-A- 0 534 867 (GIE PECHINEY RECHERCHE): Preparation of the catalyst from metallic oxides by reduction and partial carburation by reaction gases.
This application describes a catalyst for chemical and petrochemical reactions which is made up of an oxide of one of the transition metals, rare earth metals or actinide, e.g. molybdenum, comprising carbides and oxycarbides at the surface.
The manufacturing process consists in passing over the oxide the gaseous reaction mixture containing carbonaceous products which are to undergo catalytic chemical transformation at the temperature of that reaction. The carbonaceous products present in the mixture cause gradual carburation of the surface of the oxide and also a gradual increase in the efficiency of the catalyst.
The process is used, in particular, for the isomerisation of hydrocarbons for which molybdenum oxide MoO.sub.3 is a preferred catalyst. The examples describe the isomerisation of n-hexane diluted in hydrogen.
To fully appreciate the problem with which the applicants were faced, a number of characteristic parameters of the isomerisation reaction should be defined. These parameters are as follows:
Conversion rate C (in %): ratio of the number of molecules of hydrocarbon transformed either by isomerisation or by cracking to the number of molecules passed over the catalyst.
Total selectivity S tot. (in %): ratio of the number of hydrocarbon molecules isomerised to the total number of molecules transformed either by isomerisation or by cracking.
Selectivity of acyclic products S acy. (in %.): ratio of the number of hydrocarbon molecules isomerised into acyclic products to the total number of molecules transformed either by isomerisation or by cracking.
Conventional, prior art catalysts with platinum permitted higher conversion rates and higher selectivities when the molecule to be isomerised contained 6 carbon atoms (n-hexane). However, when the hydrocarbons contained more than 6 carbon atoms, beginning with n-heptane, selectivity decreased considerably, whilst the conversion rate increased ("Isomerisation of C4-C7 paraffins on zeolithic catalysts"--M. Belloum et al., Revue de l'Institut Fran.cedilla.ais du P etrole--1991 vol. 41 p. 89-107).
This phenomenon is also illustrated in FIG. 6 where the conversion rate in a stationary state is shown along the x-axis and where the corresponding selectivity of isomerised products is shown along the y-axis.
FIG. 6 requires some explanation: firstly, the conversion rates and selectivities indicated are those observed after a few hours' operation when the catalyst has reached its stationary state and the values have stabilised. Also, the conversion rate depends on the reaction conditions: mass of catalyst, gas flow, concentration of hydrocarbon in the hydrocarbon-hydrogen reaction mixture, temperature, pressure; the same is true with respect to the selectivity.
However, a very negative correlation is to be seen between selectivity and conversion, and this correlation in simple terms means that the greater the number of molecules transformed (or the more the conversion rate increases) the greater the reduction in the percentage of desirable branched products. This reduction is clearly a serious hindrance to the use of hydrocarbons above C.sub.6 as the source of branched hydrocarbons. Also, with such molecules which have a higher carbon content, cracking brings about the formation of free carbon which clogs the active surface of the catalysts and gradually makes them inefficient.
At the moment, hydrocarbons from C.sub.7-C.sub.8 cuts are transformed by catalytic gas reforming to form aromatics used as additives in petroleum to increase the octane rating. However, new legislation has imposed a dramatic reduction in the proportions of aromatics in petroleum, as explained hereinabove.
Thus, despite the fact that in the past one solution was to add aromatics, it is now no longer possible, and a satisfactory process has to be found for obtaining isomerised products from long chain hydrocarbons. A process of this kind does not exist with current catalytic processes which either have insufficient selectivity of branched isomers or a conversion yield (product of conversion rate by selectivity ) which is also inadequate to make an economical industrial process of it.
It is therefore worthwhile to find other types of catalysts which are capable of isomerising hydrocarbons of &gt;C.sub.6 cuts with high conversion rates and high selectivities of mono or multi-branched aliphatic isomers and with a minimum of aromatics and cracking carbon.